This invention relates generally to inverter systems, and more particularly to an inverter system which is operable for a predetermined period even in a case where a power failure or a service interruption occurs.
Recently, inverters operable under variable frequency are widely used for variable speed drive of electric motors. However, where a power failure of a short duration or an instantaneous power failure occurs, and the operation of, for instance, a voltage type inverter is thereby interrupted, re-start of the inverter is difficult in that the phase angle and the voltage of the output of the inverter must be matched with those of the electric motor. Otherwise, a large current flows, resulting in the failure of re-start of the inverter system.
FIG. 1 illustrates a conventional voltage type inverter system operable in a PWM (pulse width modulation) mode.
The conventional inverter system comprises a rectifier 2 for converting an AC voltage from an AC power source 1 into a DC voltage, a smoothing capacitor 3 and an inverter bridge 4 having arms formed of transistors 41-46 and diodes 41A-46A and converting the DC voltage to three-phase AC voltages. The output of the inverter bridge 4 is used to drive an AC electric motor 5. The inverter system further comprises a control circuit 14 which delivers six outputs supplied to a corresponding one of drive circuits (only one of them being illustrated) comprising a transistor 16 having its base connected to receive one of the outputs from the control circuit 14, a pulse transformer 15 through which the output (collector) current of the transistor 16 flows, and a drive circuit 17 connected to receive the output of the pulse transformer 15 and to control a corresponding one of the transistors 41-46 of the inverter bridge 4 according to the output of the pulse transformer 15.
Power required for the control circuit 14, and transistor 16 is supplied through a control power circuit PC comprising a transformer 6 connected to the AC power source 1, a rectifier 7 for rectifying the output of the transformer 6, a smoothing capacitor 8, and a chopper type constant voltage circuit composed of a transistor 9, a reactor 10, a capacitor 11, a diode 12, and a voltage control circuit 13.
The inverter control circuit 14 is illustrated in further detail in FIG. 2. The circuit 14 includes a frequency setting device 100 comprising a variable resistor connected to the constant DC voltage from the control power circuit. Operational amplifiers 102, 103, 104, resistors 101, 106, 107, 108, 109, a capacitor 110, and a variable resistor 105 in combination constitute a well-known acceleration/deceleration restricting circuit, wherein the variable resistor 105 is used for adjusting the rate of change of the frequency. The output signal f* of the frequency control circuit is converted by a V/F converter 111 into a frequency-indicative voltage signal f.sub.s which is applied to one input of a frequency synthesizing circuit 112. A triangular wave generating circuit 113 generating a triangular wave e.sub.t to be used for PWM, is applied to another input of the frequency synthesizing circuit 112. The synthesizing circuit 112 produces PWM signals based on the signals f.sub.s and e.sub.t, and controls the conducting period and ON-OFF frequency of the transistors 41-46. Since the voltage V and the frequency F of the output of the inverter is controlled in a manner in which ratio V/F is maintained constant, the control is also referred to as V/F control.
With an inverter system described above, it is, in many cases, desired to continue, even when a power failure occurs, driving the AC motor. For instance, where the AC motor is used for driving a pump in a water service system, an interruption of the operation of the AC motor might cause separation of contaminating matter which has been deposited on the internal surface of water pipes, leading to contamination of water in the pipes. To avoid such contamination, the motor driving the pump should be kept running.
When an instantaneous power failure occurs with the conventional PWM controlled voltage type inverter system, the DC voltage V.sub.d across the capacitor 3 is reduced in accordance with the commutating operation of the inverter bridge 4. When the power source voltage V.sub.AC is interrupted for a time period t.sub.1 -t.sub.2 in FIG. 3, a voltage V.sub.cd across the capacitor 8 is reduced gradually as shown, whereas the power source voltage V.sub.c of the control circuit 14 is held substantially constant because of the presence of the chopper circuit. Thus, when the original value of the power source voltage V.sub.AC is recovered at the instant t.sub.2, the voltage V.sub.cd across the capacitor 8 is brought back to the normal value, and thus operation of the inverter system is continued.
However, where the time interval t.sub.1 -t.sub.2 of the power failure is comparatively long, the voltage V.sub.cd across the capacitor 8 is reduced to a level lower than the power source voltage V.sub.c of the control circuit 14, and the power source voltage V.sub.c is no longer kept constant so that the normal operation of the control circuit 14 is no longer possible.
For this reason, in the conventional voltage type inverter system operating in the PWM mode, a guaranteed power failure time (over which the system is capable of continuing operation) is set shorter than 0.5 second, and a protecting circuit is provided for interrupting the operation of the inverter upon occurrence of a longer power failure.
It is of course possible to extend the guaranteed interruption time by increasing the capacitance of the capacitors 3 and 8. However, such a solution results in increased size and cost of the inverter system.